ByN.Gopinath

AP/CSE

Unit – II

Introduction to network layers

Internetworking• An internetwork is a collection of individual networks, connected by

intermediate networking devices, that functions as a single large network.

• different kinds of network technologies that can be interconnected by routers and other networking devices to create an internetwork

Types• Local-area networks (LANs)enabled multiple users in a relatively small

geographical area to exchange files and messages, as well as access shared resources such as file servers and printers.

• Wide-area networks (WANs) interconnect LANs with geographically dispersed users to create connectivity.

• technologies used for connecting LANs include T1, T3, ATM, ISDN, ADSL, Frame Relay, radio links, and others.

ETH

IPV4 Packet HeaderVersion HLen TOS Length

Ident Flags Offset

TTL Protocol Checksum

SourceAddr

Destination Addr

Options(variable) Pad(variable)

Data

Datagram Delivery

Packet Format

IPV4 Packet header

Fragmentation and Reassembly

Fragmentation and Reassembly

Fragmentation and Reassembly

(RARP)Reverse Address Resolution Protocol

• (RARP) is a Link layer networking protocol• RARP is described in internet EngineeringTask ForceETF) publication

RFC 903• It has been rendered obsolete by the Bootstrap Protocol (BOOTP) and

the modern Dynamic Host Configuration Protocol(DHCP)• BOOTP configuration server assigns an IP address to each client from a

pool of addresses. • BOOTP uses the User Datagram Protocol (UDP)

RouterA router is a device that determines the next network point to which a packet should be forwarded toward its destination

Allow different networks to communicate with each other

A router creates and maintain a table of the available routes and their conditions and uses this information to determine the best route for a given packet.

A packet will travel through a number of network points with routers before arriving at its destination.

There can be multiple routes defined. The route with a lower weight/metric will be tried first.

Routing

Routing

Routing

Routing ProtocolsStatic Routing

Dynamic RoutingIGP (Interior Gateway Protocol): Route data within an Autonomous System

RIP (Routing Information Protocol)

RIP-2 (RIP Version 2)

OSPF (Open Shortest Path First)

IGRP (Interior Gateway Routing Protocol)

EIGRP (Enhanced Interior Gateway Routing Protocol)

IS-IS

EGP (Exterior Gateway Protocol): Route data between Autonomous Systems

BGP (Border Gateway Protocol)

Routing

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The Routing AlgorithmThe Routing Algorithm

the shortest path tree is contained in the routing table

Calculations are based on the Bellman-Ford algorithm

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The Centralized Version of the The Centralized Version of the AlgorithmAlgorithm

Cycle Node B C D E

Initial (., ) (., ) (., ) (., )

1 (1, 1) (2, 2) (3, 1) (4, 2)

A B C

D E

2

3 45

6

1 A B C

D E

2

3 4

1

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The Distributed Version

A B C

D E

1 2

3 4

5

6

Example of simple network with 5 nodes (routers) and 6 links (interfaces)The cost of all links is assumed to be 1

From A to Link Cost B 1 1 C 1 2 D 3 1 E 1 2

Routing table for A

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Advantages

• simple to implement

• low requirement in processing and memory at the nodes

• suitable for small networks

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Disadvantages

• Slow convergence• Bouncing effect• Counting to infinity problem

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Slow Convergence

A B C

D E

2

3 4

5

6

XXX

When a link breaks the routers are supposed to reestablish the routing tables

link 1 breaks

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The Bouncing Effect

link 2 breaks and A sends its routing table to B before B sends it to A

A B C

D E

3 4

5

6

XXX

1

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Counting to Infinity Problems

D E

3

A B C2

4

5

XXX

XXX

Links 1 and 6break.

A sends its old routing table before D sends the new routing table

Subnets

• Each organization assigns IP addresses to specific computers on its networks

• IP addresses are assigned so that all computers on the same LAN have similar addresses

• Each of these lans is known as a TCP/IP subnet• Any portion of the IP address can be

designated as a subnet using a subnet mask*

* Subnet masks tell computers what part of an IP address is to be used to determine whether a destination is on the same or a different subnet

Subnet Addressing

Figure 5-6

Subnet Addressing

Example 1

Suppose that the first two bytes are the subnet indicator with addresses of the form 131.156.x.x

Then, 131.156.29.156 and 131.156.34.215 would be on the same subnet.

The subnet mask would be 255.255.0.0, which corresponds to 11111111.11111111.00000000.00000000, where 1 indicates that the position is part of the subnet address and a 0 indicates that it is not.

Subnet Addressing

Example 2

Partial bytes can also be used as subnets.

For example, consider the subnet mask 255.255.255.128, which is 11111111.11111111.11111111.10000000.

Here, all computers with the same first three bytes and last byte from 128 to 254 would be on the same subnet.

Providing Addresses

• Providing addresses to networked computers

– Static addressing– Dynamic addressing

Static Addressing

• Each computer is given an address through a configuration file

• Stored on individual computers• Problems

– Moves, changes, adds and deletes– Individuals could change their own IP address– Network renumbered

• Companies do not have a good way of tracking the addresses

Dynamic Addressing

• Server supplies a network layer address automatically– Each time user logs in– For a specific lease period

• Two standards for dynamic addressing– Bootstrap protocol (bootp) - developed in 1985– Dynamic host control* protocol (DHCP), developed

in 1993

* some say C = configuration

Dynamic Addressing

• Bootp and DHCP– Software installed on the client instructs the

client to contact the server using data link layer addresses

– Message asks server to assign the client a unique network layer address

– Server runs corresponding software that sends the client its network address and subnet mask

Leasing

• Bootp or DHCP server can either:– Assign the same network layer address to the

client each time the client requests it (bootp)– Lease the network address from the next

available on a list of authorized addresses for as long as the client is connected or for a specified amount of time -- common with isps and dial-up users (DHCP)

Address Resolution

• The process of:– Translating an application layer address to a

network address (server name resolution)– Translating the server name address to a data

link layer address (data link layer address resolution)

Address Resolution

• Server name resolution– Accomplished by the use of domain name

service (DNS)– Computers called name servers provide these

DNS services• Address data base includes: server names and their

corresponding IP address

Client computer

DNS Servernetmgr.cso.niu.edu131.156.1.11

DNS Request

LAN

LAN

Internet

DNS Request

Root DNS Server for .EDU

domain

Weber State University

Northern Illinois University

DNS Request

DNS Response

DNS Response

DNS Response DNS Serversol.acs.unt.edu137.90.2.122

Figure 5-7

Address Resolution

• Data link layer address resolution– Broadcast message is sent to all computers in

its subnet • “if your IP address is xxx.Yyy.Zzz.Ttt, please send

your data link layer address”• Uses address resolution protocol (ARP)

Network Routing

• The process of determining the route a message will take through the network– Centralized– Decentralized

• Static routing• Dynamic routing• Broadcast or multicast routing

– Connectionless– Connection-oriented routing

Route and Route Table

Computer B Destination Route

A AC CD AE EF EG C

A

BC

G

E

D F

Internet Routes

UEN

WSU CanadaOther destinations

West Coast

Europe Asia

WSUDestination RouteUEN UtahOxford EuropeU of Toronto CanadaU of Singapore AsiaUC Stanford West CoastOther Other

Types of Routing• Centralized routing• Static routing (decentralized)• Dynamic routing (adaptive and

decentralized)– Distance vector– Link state

• Other types – Broadcast routing– Multicast routing

Centralized Routing• All routing decisions are made by one computer• Main routing for star and mesh topologies• Routing tables located on each computer

– Central computer sends updated tables as needed– Routing table tells the device where to send messages

• Simplicity - no wasted resources• Hardware failures or changing conditions cause

table to be out of sync

Decentralized Routing

• Each of the following types of routing fall under the heading of decentralized routing

• Each device makes its own routing decisions with the use of a formal routing protocol

• Routing protocols are self-adjusting– Can automatically adapt to changes in the network configuration

• Drawbacks– Slows down the network with status messages– Requires more processing by each computer

Static Routing

• Routing table developed by the network manager or some type of committee– Initial table sent to each computer which then

updates the routing table as needed– Reroutes as needed with down or removed

circuits– Updated when new devices announce their

presence– Used in relatively static networks that have few

routing options

Dynamic Routing (Adaptive)• Routing messages over the fastest route

– Used when there are multiple routes in the network

– Improves network performance by selecting the fastest route to avoid bottlenecks or busy circuits

– Initial table developed by network manager– Dynamically updated with changing conditions by

the devices themselves– Monitors message transmission time or each

device reports how busy it is to avoid bottlenecks• Disadvantages

– Requires more processing by each computer– “Wastes” network capacity

Dynamic Routing (Adaptive)• Distance vector dynamic routing

– The number of hops along a route – Exchange information with the neighboring computers every few

minutes

• Link state dynamic routing– The number of hops along a route– The speed of the circuits on the route– How busy the route is– Exchanges information with other routing devices every 15-30

minutes– Tries to determine the fastest route– Converges reliable routing information more quickly

Routing Protocols

• RIP, IGP, OSPF, EGP, BGP• Distance vector routing protocols (RIP,

Appletalk,IPX, IGRP)– Routers inform neighboring routers of table– Closest router is used to route packets

• Link State routing protocols (OSPF)– Routers have at least a partial map of the network– Changes are flooded throughout network– Routes are recomputed

Interior and Exterior Routing

• Interior routing is within an autonomous system (collection of routers under a single administrative control)—RIP, OSPF

• Exterior routing occurs between autonomous systems

• Network access protocols operate at Layer 2. – Transport of IP datagrams– IP over point-to-point connections is used by ISP

when you dial in

Routing Protocols• Border Gateway Protocol• Internet Control Message Protocol • Routing Information Protocol• Open Shortest Path First• Enhanced Interior Gateway Routing

Protocol

Routing Protocols• Internet protocols

– BGP (border gateway protocol)• Exchanges information between autonomous systems

about the condition of the internet• Complex, hard to administer, exterior routing protocol

– ICMP (internet control message protocol)• Simple, interior routing protocol used with the internet• Reports routing errors but is limited in the ability to

update – RIP (routing information protocol)

• Dynamic distance vector interior routing protocol• Counts the number of devices on each route• Selects the route with the least number of devices

Routing Protocols– OSPF (open shortest path first)

• Link state interior routing protocol used on the internet• Counts number of computers, network traffic, network error

rates to select the best route• Doesn’t broadcast to all devices just to routing devices• Preferred TCP/IP, but also used by IPX/SPX

– EIGRP (enhanced interior gateway routing protocol)• Link state interior routing protocol developed by CISCO• Uses route transmission capacity, delay, reliability and load

to select best route• Stores multiple routing tables

– SAP (service advertisement protocol)• Netware servers send SAP advertisements• Novell’s broadcast protocol

Broadcast Routing

• Sends the message to all computers on the network

• Only computer with correct address processes the message

• Used only in bus networks• Wastes network bandwidth

Multicasting– Similar to broadcasting – Only works within one LAN or subnet– Messages sent from one computer to another on the network

is called a unicast message– Messages sent to a group of computers is called a multicast

message– Targeting a specific work group – IGMP (internet group management protocol)

• Sends an IGMP multicast request to the routing computer• Assigned a special class D IP address to identify the group• The routing computer sets the data link layer address• All participating machines will process messages sent to this

address• Sends a IGMP message notifying of end of session

Thank u